When deep lung dives are to be undertaken, decompression requirements make it necessary for the diver to remain under the surface of the water for some considerable time. Open circuit breathing is therefore no longer feasible because of cost, and it is necessary for a diver to rebreath atmosphere which has already been breathed, scrubbing the carbon dioxide exhaled by the river with a suitable material (for example barium hydroxide) and replenishing the oxygen as it is required. It is believed that the safest inert gas to be mixed with oxygen is helium, and for deep dives it is necessary that the percentage of oxygen be very much less than under atmospheric conditions. Thus typically it is necessary for the oxygen content of the breathable gas to be about 10% if the depth of the dive is between 250 feet and 600 feet. (The standard procedure is to use air 0-100 feet; from 100-250 feet either 50% helium, 50% air or 20% O.sub.2 in helium; from 600 feet to a greater depth the oxygen content is varied in accordance with calculations, but will be less than 10%). If oxygen is used to excess, oxygen poisoning can result, this being a most serious hazard to a diver.
In the U.S. Pat. No. 3,556,098 issued to John W. Kanwisher and Walter A. Starck there was described and claimed an apparatus for use by divers wherein the partial pressure of the oxygen in the breathing gases was sensed and when the partial pressure dropped below a lower limit which was not predetermined for a specific dive, but calculated as a percentage per atmosphere, an electrical circuit was energised to drive a solenoid and to add further oxygen. Equipment produced in accordance with the said Patent Specification has been used with some degree of success, but certain difficulties have been encountered. Owing to the lethal nature of difficulties encountered with diving apparatus it is not clear what the basic problems have been, but it is believed that one of the difficulties encountered is the inability of partial pressure sensing to provide a very smooth transition from one mixture strength to another, (for example, when surface supplied emergency gas needs to be used). Furthermore, the percentage of oxygen in the breathing gases varies widely with differences of pressure encountered under normal diving procedures. It is recognised that there are some circumstances under which the human body can tolerate only gradual changes (not more than 3% per minute), and one of the objects of this invention is to provide means whereby the oxygen is maintained, not to pre-determined partial pressure, but to a pre-determined percentage of the total breathable gas.
Further, to meet the difficulty of a diver transferring from automatic to manually controlled fixed percentage breathable gases, another object of this invention is to provide simple means whereby a diver can control the atmosphere which he breathes, over-riding the automatic control device.
Other prior art includes the United States Specification accompanying the U.S. Pat. No. 3,695,261 issued to Donald R. Emmons, wherein a re-breathing apparatus was described which was useful for scuba diving, and the apparatus incorporated means for breathing into and out of a CO.sub.2 scrubber for the removal of CO.sub.2, and an oxygen sensor for sensing the partial pressure of oxygen in the atmosphere of the re-breather. It is stated to be not useful for very deep dives, (say exceeding 200 feet) except for very short periods of time, partly because the device described therein does not provide means for breathing inert gases other than nitrogen, and partly because it does not provide means for reducing the percentage of oxygen (by volume) as the diver depth is increased, and oxygen poisoning or nitrogen narcosis can result.
Further prior art known to the applicant includes the specification accompanying U.S. Pat. No. 3,252,458 issued to A. R. Krasberg. In that apparatus however the sensing of oxygen was again the sensing of partial pressure rather than the sensng of percentage.